The present invention relates to mast staging of a multistage mast assembly, in particular to method and apparatus which reduces shock loads encountered in the transition between stages of a multistage mast assembly of a lift truck
Known lift trucks, such as Reach-Fork® and Swing-Reach® trucks available from The Raymond Corporation in Greene, N.Y., include a vertically extendible mast supporting a carriage which incorporates “mast staging” to lift a carriage to considerable heights. “Mast staging” refers to a method of lifting/lowering loads on a lift truck carriage in stages (sections). To lift, a fixed hydraulic ram extends until it reaches its end of stroke, whereupon, successive rams (stages) continue the lift. Unfortunately, a shock load is generated as one ram decelerates rapidly at its end of stroke and the next ram accelerates upward. These shock loads reduce the life of the mast components, and can propagate through the lift truck to the discomfort of the operator, and can destabilize on the carriage.
One known method of mast staging intended to reduce shock loads is disclosed in U.S. Pat. No. 5,022,496. The disclosed method continuously monitors the carriage position relative to the mast, and slows the rate of movement of the carriage immediately before and during a stage transition. Once the transition is complete, the carriage speed is increased to the operator selected speed. This method changes the speed of the carriage as it moves in a vertical direction which can destabilize a load on the carriage, and cause discomfort to an operator.
Another known method of mast staging intended to reduce shock loads is disclosed in U.S. Pat. No. 5,657,834. The disclosed method incorporates spring elements at the end of each mast stage to cushion the transition between stages. This particular method increases the complexity of the mast assembly and the difficulty of retrofitting an existing mast assembly. Therefore a need exists for a mast staging method and apparatus which can be easily incorporated into a lift truck, and does not require changing the vertical speed of a carriage during the transition between stages.